The present invention deals with disc drives. More particularly, the present invention deals with a device for retaining an axial load in a hub assembly of a disc drive.
A typical disc drive includes one or more magnetic discs mounted for rotation on a hub or spindle. Where more than one magnetic disc is used, the discs are spaced apart from one another axially along the hub by spacers mounted between the discs. Conventional hubs typically include a flange portion which extends from one of the axial ends of the hub. The discs and spacers are placed concentrically about the hub and are supported by the flange portion of the hub. The plurality of magnetic discs and the spacers are clamped down onto the flange portion of the hub using a clamp which is placed on the axial end of the hub, opposite the flange. Thus, the discs and spacers are all clamped to the hub for rotation with the hub about an axis of rotation generally defined by the radial center of the hub.
A typical magnetic disc drive also includes a transducer supported by a hydrodynamic air bearing which flies above each magnetic disc. The transducer and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls the disc drive to retrieve information form the magnetic discs and to store information on the magnetic discs.
An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored on the magnetic discs by providing a write signal to the data head to encode flux reversals on the surface of the magnetic disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the magnetic disc, sensing the flux reversals on the magnetic disc, and generating a read signal based on those flux reversals. The read signal is then decoded by the drive controller to recover the data represented by flux reversals stored on the magnetic disc, and consequently represented in the read signal provided by the data head.
As industry pressure requires disc drives to be reduced in size, the axial height of the stack assembly, and consequently the axial height of the entire disc file, becomes critical. In past systems, the clamp used to hold the discs about the hub was located above the hub and screwed onto the hub with screws running in the axial direction. However, since the axial height of the stack assembly has become critical, the clamp and the screws used to fasten the clamp to the hub take up an undesirable amount of axial space.
Therefore, a heat shrink clamp was developed. Such a clamp is described in greater detail in U.S. Pat. No. 4,639,802. Such clamps typically include a clamp ring which has an inner diameter that is slightly smaller than the outer diameter of one axial end of the hub. The clamp ring is responsive to thermal energy and expands when thermal energy is applied to it and contracts when thermal energy is removed from it. Therefore, to assemble the clamp ring onto the hub, the clamp ring is first heated, thereby expanding it such that the inner diameter of the clamp ring is slightly larger than the outer diameter of the hub. The clamp ring is then placed about the hub and allowed to cool to establish a frictional or interference fit with the outer surface of the hub.
After the plurality of discs and spacers are arranged about the hub, and after the clamp is placed on the hub, the assembly is subjected to an axial load, referred to as the "preload". Under the preload, the clamp ring is allowed to cool and form its frictional fit with the hub before the preload is removed. The clamp ring retains a portion of the preload and thus clamps the spacers and the magnetic discs to the flange located at the second axial end of the hub.
A major concern which arises in designing a magnetic disc hub assembly is the manner in which the magnetic discs are clamped together. The discs must be clamped with sufficient axial force to prevent radial movement of the discs which could result from unbalanced rotational forces, thermal expansion, or shock or impact loads during product shipping. To ensure that there is a minimum radial shifting of the discs, the stack of discs must be subjected to a specified clamping force which is exerted by opposing axial forces applied to the discs from the clamp ring and the flange.
The clamp ring is usually made of the same material as the hub. Once the preload is removed, the clamping force exerted by the clamp ring is relaxed. There are a number of reasons for the relaxation of the clamping force. For example, when the preload is applied, the discs are slightly compressed. When the preload is removed, both the hub and the clamp ring are slightly stretched in the axial direction by the compressed discs which tend to resume their original shape after the preload is removed. If the clamping force is relaxed beyond the minimum force required to prevent radial disc movement, the hub assembly will not meet specification and must be discarded.
In addition, the clamping force in traditional hub assemblies can vary with temperature changes in the air surrounding the hub assembly. While the clamp ring and the hub are made of the same material, the discs and the hub are traditionally not made of the same material. Thus, there is typically a mismatch between the thermal expansion coefficients of the discs and the hub. When the air surrounding the hub assembly undergoes a significant temperature change, the mismatch in expansion coefficients can result in a substantial change in the clamping force clamping the discs to the flange. This can cause the clamping force to drop below the minimum force requirement for preventing disc radial movement on the hub assembly.
While some attempts have been made at introducing a resilient mechanism into the hub assembly (such as that set out in U.S. Pat. No. 5,006,942) such devices can be expensive and inefficient in retaining preload. Thus, there is a continuing need for devices in the hub assembly which provide a mechanical buffer for storing and retaining physical strain exerted by the preload in order to compensate for loss of preload by stretching of the hub and clamp ring, and by thermal expansion differences between the hub material and the material used in making the discs.